The production of glass-ceramic articles originated in U.S. Pat. No. 2,920,971. As is explained therein, glass-ceramic articles are derived through the controlled heat treatment of precursor glass bodies. Hence, the preparation of glass-ceramic articles commonly contemplates three basic steps: first, a glass forming batch of a desired composition, most frequently containing a nucleating agent, is melted; second, the melt is simultaneously cooled to a glass and an article of a predetermined configuration is shaped therefrom; and, third, the glass article is subjected to a heat treatment whereby nuclei are first generated in situ within the glass and thereafter crystals are grown on those nuclei.
Because the crystals are formed on a myriad of previously-developed nuclei, the microstructure of glass-ceramic articles typically consists of relatively uniformly-sized, fine-grained crystals homogeneously dispersed throughout a residual glassy matrix. Inasmuch as glass-ceramic articles are generally highly crystalline, viz., greater than about 50% by volume, the mechanical strengths thereof will normally be substantially greater than those of the precursor glass bodies. In point of fact, the glass-ceramic product will customarily exhibit physical properties quite different from those of the parent glass and more closely akin to those of the crystal phase. For example, where a refractory crystal phase is developed, the glass-ceramic will typically have a higher use temperature than that of the initial glass. The residual glassy matrix will generally have a composition very different from that of the precursor glass body since the components comprising the crystal phase will have been removed therefrom. Finally, because the crystals are grown in situ and are dispersed within a continuous residual glassy matrix, glass-ceramic articles are free from voids and non-porous.
In the more than two decades that have elapsed since the initial disclosure of glass-ceramic articles, many workers have entered the field and their research has led to the production of glass-ceramic bodies from a broad range of parent glass compositions. This capability of preparing glass-ceramic bodies from vastly different starting materials has resulted in products of widely-varying properties which, in turn, has recommended their utility in a diverse assortment of applications.
As noted above, certain glass-ceramic products demonstrate high temperature capabilities which, when coupled with a relatively low coefficient of thermal expansion to insure good resistance to thermal shock, have suggested their use in such applications as preform cores in the making of hollow metal castings, such as jet engine blades and vanes.
Although glass-ceramic articles are commonly inherently mechanically stronger than glass, for some applications even higher strengths are demanded. That requirement has led to the development of means for enhancing the strength of glass-ceramic bodies through such techniques as thermal tempering, chemical strengthening, and laminating to implant a thin surface layer thereon having a lower coefficient of expansion than the interior portion. Another method for imparting increased strength to a glass-ceramic article, which also appears to improve the toughness thereof, involves the incorporation of inorganic fibers therein.
U.S. Pat. No. 3,607,608 illustrates the inclusion of fibers prepared from stainless steel, boron, SiC, or graphite into glasses, carbides, nitrides, Al.sub.2 O.sub.3, and devitrified glasses. In the latter case, the fibers are aligned in a molten glass, the mass pressed together into a glass body of a desired geometry, and the glass thereafter devitrified (crystallized) through exposure to a heat treatment. Two base glass compositions were reported in the patent and those are listed below in weight percent:
______________________________________ SiO.sub.2 4 SiO.sub.2 28.7 Al.sub.2 O.sub.3 3 CaO 9.1 B.sub.2 O.sub.3 10 Na.sub.2 O 11.77 PbO 83 B.sub.2 O.sub.3 26.3 ZnO 5.3 BaO 17.2 F.sub.2 3.1 ______________________________________
U.S. Pat. No. 3,681,187 describes the incorporation of carbon fibers into a variety of glass and glass-ceramic bodies. The patent cites the use of glass-ceramics having base compositions within the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2, Li.sub.2 O-ZnO-SiO.sub.2 -P.sub.2 O.sub.5, and Li.sub.2 O-MgO-SiO.sub.2 -P.sub.2 O.sub.5 systems. The fibers may be aligned in molten glass or hot pressed together with powdered glass. In the latter practice, the pressing is carried out at a sufficiently high temperature to cause the glass to become plastic and flow around the fibers to form a proper matrix therefor. The resulting glass composite was thereafter heat treated to effect crystallization in situ of the glass.
U.S. Pat. No. 3,940,277 outlines a method for making glass-ceramic articles exhibiting toughness and thermoplastic characteristics composed of silica fibers dispersed within a glassy matrix having a base composition in the Na.sub.2 O and/or K.sub.2 O-SiO.sub.2 field.
U.S. Pat. No. 3,948,669 is directed to the production of glass-ceramic articles containing TiO.sub.2 fibers which are grown in situ via heat treating glasses having base compositions within the alkaline earth metal oxide-Al.sub.2 O.sub.3 -B.sub.2 O.sub.3 -TiO.sub.2 system.
The high temperature capability of SiC has recommended the use of fibers of that composition as reinforcing agents in applications wherein the articles will be exposed to very elevated temperatures. U.S. Pat. Nos. 3,161,473 and 3,371,995, for example, specifically refer to the use of such fibers in glasses and ceramics.
Other patents of interest include U.S. Pat. Nos. 4,256,378, 4,263,367, and 4,265,968 which are directed to the use of graphite fibers as reinforcing elements in glass composite materials; U.S. Pat. No. 4,314,852 which discloses the utility of SiC fibers as reinforcing agents in glass composite materials; and U.S. Pat. No. 4,324,843 which describes the applicability of SiC fibers for reinforcing ceramic composite materials.
One problem that has been experienced in the use of SiC fibers as reinforcing agents in matrices which will be exposed to very high temperature environments (.about.1000.degree. C. and higher) has been the propensity of the fibers to be oxidized and, therefore, subject to disintegration with consequent loss of mechanical strength in the composite article.
Accordingly, the primary objective of the instant invention is to provide a glass-ceramic body of high strength and capable of high temperature use, that is, capable of use in applications involving exposure to temperatures in excess of 1000.degree. C. and up to 1200.degree. C.
A more narrowly-defined objective is to provide a glass-ceramic material capable of high temperature use and which demonstrates excellent resistance to oxidation at temperatures up to 1200.degree. C. This latter feature permits the material to be utilized as a matrix to be reinforced through the incorporation of SiC fibers therewithin, since it will protect the fibers from disintegration through oxidation thereof.